The present invention relates generally to a metal patching method and composition and more particularly to a two component system which is heat cured, the composition changing color when curing is complete. Various metal patching systems are known in the art. Such systems are used, for example, to seal refrigerator evaporators and patch cracks in metal surfaces of all types. Two types of metal patching materials are generally employed. One type is a two component system, the components being mixed together at room temperature to initiate a reaction which leads to curing or polymerization of the batch composition. A second type employs a single stick in which curing is initiated by the application of heat.
Room temperature cured epoxy systems, such as bisphenol A and an amine system have several disadvantages. For example they are usually made up of relatively short polymer chains which are weaker than the longer polymer chains of heat cured resins. Room temperature cured epoxies also have the disadvantage of being relatively liquid prior to curing so that when applied to the area to be patched, they would flow and not remain in place and consequently it was difficult to patch an overhead or vertical structure. Perhaps the primary disadvantage of room temperature cured epoxies is their relatively long curing time of three to four hours. Some newer epoxy systems employing bisphenol A with mixtures of amines and anhydride curing agents claim a room temperature cure in 5 to 15 minutes. However the cure is only 60 to 70% complete in this time and it still takes up to four hours for the reaction to go to completion.
While heat cured epoxy resins are substantially stronger than the room temperature cured epoxies, they too have several disadvantages. For example they cure very rapidly and therefore it is difficult to control the curing. A typical heat cured system comes in the form of a stick and includes cold mixed bisphenol A and an anhydride curing agent. The metal to be patched is heated and the stick wiped over the heated area. If too much heat was applied to the metal or the heat applied for too long a period, curing is rapid and an overcured or even crystalline structure will result which produces a poor patch. On the other hand too little heat or heat applied for too short a time period will result in an undercured, relatively weak patch.
According to the present invention, however, a two component, heat cured system is provided in which a controlled heat curing cycle is made possible by a color change which occurs in the composition when curing is substantially complete. This guarantees a perfect cure and therefore maximum bond strength. Further, unlike the two component systems of prior art, the present invention is easy to use, stays in place and will not run off during curing even when applied to overhead or vertical structures.